Group III nitride crystal substrates such as AlxGayIn1-x-yN crystal substrates are significantly useful as substrates for various semiconductor devices such as light-emitting elements, electronic elements, and semiconductor sensors. Here, in order to improve properties of the various semiconductor devices, there is a demand for AlxGayIn1-x-yN crystal substrates each having a low dislocation density and favorable crystallinity. Further, from a viewpoint of utilization efficiency of the AlxGayIn1-x-yN crystal substrates, a main plane of each of the substrates is required to have an area of at least 10 cm−2, preferably at least 20 cm−2.
Therefore, to fabricate the AlxGayIn1-x-yN crystal substrates each having a large size and a low dislocation density, various methods have been proposed in X. Xu and five others, “Growth and characterization of low defect GaN by hydride vapor phase epitaxy” J. Crystal Growth, 246, (2002), p 223-229 (hereinafter referred to as Non-Patent Document 1), Japanese Patent Laying-Open No. 2001-102307 (hereinafter referred to as Patent Document 1), and others.
Non-Patent Document 1 discloses that in growth of a GaN crystal, for example, a dislocation density of the GaN crystal is decreased as a thickness of the grown crystal is increased. However, in such a method of decreasing a dislocation density by increasing the thickness to be grown, even if a GaN crystal is grown to have a thickness of 3 mm, it is difficult to decrease the dislocation density to 1×106 cm−2 or lower, and hence only a small effect of reducing dislocations is obtained. Furthermore, in such a method of decreasing a dislocation density, variations in dislocation density in a substrate plane also cause variations in dislocation density of a GaN crystal, and hence a region having a high dislocation density may remain in the GaN crystal.
Patent Document 1 discloses a method of forming multiple pits each having a minute inclined plane, on a crystal growth plane, in growing a GaN crystal, and causing dislocations to occur intensively in these pits to thereby reduce dislocations in a region other than the pits. However, in such a method of decreasing a dislocation density, multiple pit regions each having a high dislocation density remain in a Group III nitride crystal.
Therefore, an AlxGayIn1-x-yN crystal substrate having a large size and a low dislocation density and suitably used for semiconductor devices has not yet been heretofore obtained.    Patent Document 1: Japanese Patent Laying-Open No. 2001-102307    Non-Patent Document 1: X. Xu and five others, “Growth and characterization of low defect GaN by hydride vapor phase epitaxy”, J. Crystal Growth, 246, (2002), p 223-229